Surface followers and measuring apparatus of the general type to which this invention relates are described in the U.S. Pat. Nos. 1,946,924 Allen; 3,194,055 Knobel; 3,321,838 Albertson; 3,495,422 Rejsa, and 3,528,002 Dunlavey. Other background disclosures are contained in British Pat. Specification No. 839,996 (1960); USSR Inventors' Certificates 589,545 (1978) and 603,844 (1978), Japanese Pat. Application Public Disclosure 52-38244 (1977) and Swedish application No. 7900795-1, (published 1980).
Pneumatic surface followers are frequently used to maintain a predetermined distance between two objects. One or both objects may carry a transducer element to provide an input to a system for effecting measurement and control.
The thickness or caliper of moving sheets is commonly measured by passing the sheet over a roll or fixed plate on one side of the sheet while a surface follower automatically positions itself at a nominally constant distance from the surface of the sheet on the other side. Either the plate or the surface follower or both may contain a proximeter element such as a magnetic reluctance or eddy current sensor element while the other may contain a target element. An unsupported sheet may travel between two surface followers, one of which may contain a proximeter while the other may contain a target element. The proximeter responds to its distance from the target, and the sheet thickness is derived in effect by subtraction of the nominally constant distance or distances of the sheet follower or followers from the sheet surface or surfaces. It has been found, however, that the nominally constant distance is subject to variation with changes in the gas supply pressure.
Moreover the nature of the forces acting on the body, together with its mass, have generally resulted in a dynamic system with a high degree of instability, so that the position of the body tends to go into oscillation in response to slight upsets such as rapid changes in the thickness or the spatial position of the sheet being measured. Most, if not all, pneumatic surface followers have a passage that carries the flow of pressurized gas to be discharged against the sheet surface, and commonly the pressure in the passage varies with the distance of the surface follower body from the surface. Commonly this pressure variation produces what is herein termed "positive feedback", in that a variable force component acting on the surface follower body tends to move it, or allow it to move, farther from the surface when the pressure increases and closer to the surface when the pressure decreases.
The positive feedback may be accidental or introduced by deliberate design, on the theory that when the surface follower body is too close to the surface, for example, the gas outlet is abnormally restricted, causing the gas pressure in the passage to increase abnormally, and that the correct action to take is to use the abnormally increased pressure to generate a force, or increased force, causing the body to move farther from the surface, and thus tending to correct the original error in the position of the body.
It now appears that such prior art has failed to recognize that when the surface follower body is levitated by a "gas bearing" film of pressurized gas, in the above situation the increased levitating force generated by the increased gas bearing pressure may itself be more than sufficient to move the body away from the surface to its correct normal distance therefrom. Hence the addition of the positive feedback force component may simply help to drive the body too far from the surface and hence force the system into undesired oscillation. In other words, it appears that the prior art has failed to recognize that the positive feedback force component is applied in the wrong direction, or with the wrong phase, and that what is actually needed is what is herein termed "negative feedback" in order to stabilize the system.